1. Field of the Invention
The present invention relates generally to pick-and-place devices for picking up electrical components from source locations and placing them onto printed circuit boards (PCBs), and more particularly to a method and apparatus for positioning the electrical components for pick-up by a pick-and-place device.
2. Description of the Prior Art
Pick-and-place devices are used in automated PCB assembly to pick up particular electrical components known as surface mount devices (hereafter "SMDs") from source locations and to place the SMDs on a PCB in predetermined mounting locations.
An SMD is an electrical component having a molded body and a plurality of fixed leads extending from the molded body. An integrated circuit is typically mounted in the molded body, and electrical signals are transmitted to and from the integrated circuit through the fixed leads. The fixed leads are typically connected to an exterior circuit mounted on a PCB. A commonly-recognized SMD is a dual-in-line package ("DIP") which has two rows of fixed leads disposed on two parallel sides of the molded body. In another SMD design, several rows of fixed leads extend from a lower surface of the molded body. Although the number and location of leads and the size of the molded body varies, all SMDs are similar in that their molded body is formed with a flat upper surface which can be accessed by a pick-and-place device as described below.
Pick-and-place devices typically include a computer-positioned pick-up tool which picks up an SMD from one of a plurality of source locations and places the SMD on a PCB at a predetermined mounting location.
As shown in FIGS. 9 through 11, a typical prior art pick-and-place device 10 includes a first carriage 20 for positioning a pick-up tool 30 in an X-direction, a second carriage 40 for positioning fixtures 50 in a Y-direction, and a third carriage 60 for positioning a PCB 70 in the Y-direction. In addition, a mechanism (not shown) is disposed on first carriage 20 for moving pick-up tool 30 in the Z-direction toward and away from fixture 50 or PCB 70. Connected to fixture 50 are one or more SMD storage tubes 80. SMDs are fed from storage tubes 80 into a pick-up position 51 formed in each fixture 50.
Other embodiments of pick-and-place device 10 can be used. For example, fixtures 50 can be stationary, and first carriage 20 can be disposed to move in the Y-direction, along the row of pick-up positions, as well as the X- and Z-directions.
A pick-and-place operation using pick-and-place device 10 will now be described with the aid of FIGS. 9-11. First, second carriage 40 is moved in the Y-direction until a selected pick-up position 51 of fixture 50 is aligned with the X-direction movement of pick-up tool 30. In addition, third carriage 60 is moved in the Y-direction until a selected mounting location 71 of PCB 70 aligns with the X-direction movement of pick-up tool 30. First carriage 20 is then moved in the X-direction until pick-up tool 30 is disposed over selected pick-up location 51. Pick-up tool 30 is then moved in the Z-direction toward the pick-up location 51. An SMD disposed in the selected pick-up location 51 is then secured to an open end 31 (FIG. 11) of pick-up tool 30 by moving open end 31 to contact the flat upper surface of the SMD, and then generating a partial vacuum in the pick-up tool 30 such that the SMD is forced against open end 31 of the pick-up tool 30 by suction. The SMD is then lifted from the selected pick-up location 51 by the Z-direction movement of pick-up tool 30. First carriage 20 then moves in the X-direction to position pick-up tool 30 over selected mounting location 71. Pick-up tool 30 is then moved in the Z-direction until the SMD is placed in selected mounting position 71. Finally, the partial vacuum in pick-up tool 30 is terminated, thereby releasing the SMD from pick-up tool 30.
During the pick-and-place operation, a successful SMD pick-up is determined by sensing the partial vacuum in pick-up tool 30, which indicates that an SMD is blocking open end 31. That is, if an SMD is not picked-up, then a vacuum cannot be maintained in pick-up tool 30. When it is determined that an SMD was not picked-up, pick-up tool 30 typically moves to pre-programmed alternate pick-up location from which a desired SMD can be obtained. When no vacuum is detected for any of the alternate pick-up locations, the pick-and-place operation is terminated until the SMDs are manually supplied to the pick-up locations and a reset signal is activated.
As mentioned above, SMDs are positioned for pick-up using fixtures 50 into which SMDs are fed from storage tubes 80.
As shown in FIGS. 12 to 14, a typical prior art fixture 50 is precision-machined from an aluminum block. Fixture 50 includes a base 52 defining one or more nests 53 bounded by side walls 54. Nests 53 extend from a first end 55 of base 52 to a front wall 56. The width and length of nests 53 are determined by the size of the SMD to be positioned for pick-up. Pick-up location 51 is located in nest 53 adjacent front wall 56. Also formed on base 52 is a hole 57 which is used to connect base 52 to second carriage 40. Finally, a storage tube receiving lip 59 is formed in nest 53 adjacent first end 55. Lip 59 is used for receiving an end of a storage tube.
As shown in FIGS. 15 to 17, prior art SMD storage tube 80 typically includes a bottom wall 81, side walls 82 and an upper wall 83. As shown in FIG. 17, a longitudinal axis L is defined by a length of the storage tube 80. A passage 84 is defined within storage tube 80 by bottom wall 81, side walls 82 and upper wall 83. SMDs 90 are confined to slide in passage 84 along longitudinal axis L. Typically, a clearance is formed within storage tube 80 such that SMDs 90 can move slightly from side to side and in a vertical direction in passage 84.
As shown in FIG. 16A and 16B, bottom wall 81 of storage tube 80 can either be flat or can include a centering ridge 85. Ridge 85 is disposed on bottom wall 81 along the longitudinal axis L. SMDs 90 disposed in storage tube 80 straddle ridge 85 such that the SMDs are centered in passage 84. Ridge structures, other than the "W-shaped" structure of FIG. 16B, can also be used for the purpose of centering SMDs 90.
As shown in FIG. 17, SMDs 90 are typically aligned in storage tube 80 such that they abut each other along longitudinal axis L. A first end 86 and a second end 87 of storage tube 80 are typically covered or blocked during shipping such that the SMDs 90 do not slide out of storage tube 80 through first end 86 or second end 87.
As shown in FIG. 18A, the prior art method of positioning SMDs for pick-up by pick-up tool 30 includes connecting storage tube 80 to a fixture 50 such that SMDs 90 slide from storage tube 80 into nest 53. Storage tube 80 is supported in an inclined position such that end 86 of storage tube 80 rests against lip 59 of fixture 50. Lip 59 is typically sized such that SMDs 90 slide smoothly from bottom wall 81 into nest 53. A vibrating mechanism (not shown) is typically located between the second carriage 40 and the fixture 50 to shake the SMDs from thee storage tube 80 into nest 53.
Also shown in FIG. 18A is an SMD 90 correctly located in nest 53 at pick-up location 51. During normal operation, when SMDs are caused to slide into nest 53 from storage tube 80, lead SMD 90 is pushed by the remaining SMDs 90' into pick-up location 51. When properly located in pick-up location 51, an upper surface 91 of SMD 90 is positioned parallel to a lower surface of nest 53. In this position, SMD 90 can be picked up by pick-up tool 30.
There are several disadvantages associated with the prior art positioning method, described above.
First, the prior art method is expensive to implement because each fixture 50 is precision-machined from aluminum blocks. Precision machining is expensive because it requires expensive machine tools and substantial amounts of skilled labor.
Second, each fixture 50 can only be used for SMDs of a single size. Because SMDs having numerous sizes are typically mounted onto a single PCB, numerous fixtures 50 must be produced to position all of the SMDs for pick-up.
As shown in FIG. 18B, a third disadvantage occurs because the SMDs slide from passage 84 to nest 53. This requires each SMD to slide from one surface, inside storage tube 80, to a second surface, inside nest 53. When storage tube 80 is not fitted properly to fixture 50, a gap is formed between the two surfaces. This gap often causes SMDs to become snagged or trapped, as shown in FIG. 18B.
As shown in FIG. 18C, another problem is caused when SMDs are not smoothly fed into nest 53. Because of the transition between storage tube 80 and nest 53, the forces exerted by adjacent SMDs can become misaligned. These misaligned forces can cause SMD 90 to "piggy-back" onto an adjacent SMD 90', as shown in FIG. 18C.
As shown in FIG. 19, another disadvantage is that prior art fixtures 50 use an excessive amount of space on second carriage 40. As shown, second carriage 40 has a maximum length d along which fixtures 50 can be disposed. When fixtures 50 are disposed on second carriage 40, a portion e of the length d is taken up by a spacing between each pair of fixtures 50. In addition, portions f are taken up by side walls 54 of each fixture 50. Finally, a portion g is taken up by unused nests 53. The cumulative unused space taken up by portions e, f and g greatly reduces the usable portion of length d. This greatly reduces the number of pick-up positions which can be disposed on second carriage 40. Therefore, the number of pick-up positions 51 is not optimized using the prior art method.